The present invention relates to an improved balance on which strain gauges are mounted for determining the forces and moments on an aircraft model in a wind tunnel.
There are two general methods of wind tunnel testing of an aircraft model. One, known as sting mounted, is to suspend a model of the aircraft in the center of a wind tunnel by a supporting member which contains the strain gauge instrumentation required to measure the forces and moments generated by the aircraft model in reaction to the air flow. The member supporting the model must be as small as possible so that the forces exerted on the support structure interferes to a minimum extent with the measurement of the forces exerted on the model itself.
The second method, which is the one used in this preferred embodiment, is to attach one half of an aircraft model to a structure near the floor of the wind tunnel by an instrumented balance. This arrangement has two advantages. First, since the model should not occupy more than approximately 1 and 2 percent of the cross sectional area of the wind tunnel, and since only one half of the model is being used, the model scale may be 1.4 times as large for the same blockage area. Second, the balance and other supporting hardward for the model is not in the wind stream and therefore may be constructed in any size and shape that is conducive to the accurate measurement of the aircraft model forces and without regard to the effect of the windstream on the support members themselves.
A unique condition associated with the testing of this type of aircraft model, known as a reflection plane wind tunnel model, is that the force generated by the fuselage half produces what appears to the balance to be a side force. Of course, in the actual aircraft, there would be no side force since, by definition, both halves of the aircraft model would be identical and the resultant side forces would cancel. The instrumentation therefore need not measure this force.
Another general consideration is that the balance flexures, beams and strain gauges should be configured so that a minimum number of strain gauges supply all the data, thereby simplifying and making more accurate the subsequent data reduction.
A variety of balances have been constructed, many capable of measuring the three forces and three moments produced by an aircraft model. However, measuring all six forces and moments represents an unneeded level of complexity for a reflection plane wind tunnel model since the side force is disregarded. Five component balances have been built in a rectangular orientation with beams and flexures in each corner of the rectangle. However, this configuration is not efficient since the large rolling moment reaction forces are felt by all of the balance gauged members, making the separation and measurement of the other forces more difficult.
What is required for the accurate testing of a reflection plane wind tunnel model is a balance optimized for the simple and accurate measurement of the five pertinent components with a minimum of instrumented beam elements to faciliate a simple and accurate data measurement and reduction process.